High-frequency transistors have been widely used for the purpose of, for example, high-frequency signal processing such as signal transmission, signal reception, and switching received signals and the like of wireless communication apparatuses such as a mobile phone and a wireless LAN.
For such a high-frequency transistor, a small coupling capacitance (a small parasitic capacitance) with respect to a substrate is demanded in order to perform high-speed operation.
Therefore, as a high-frequency transistor, an FET (field-effect transistor) of a compound semiconductor such as GaAs with low electric power consumption capable of simply forming a complicated circuit has been often used for a long time.
However, in a compound semiconductor-based device such as a GaAs-based device, the material cost is higher than that of a silicon-based device due to the material of a substrate and difficulty in increasing a diameter of a substrate. Further, in response to request of achieving small-sized wireless communication apparatuses and system-on-chip, it is increasingly demanded to form such a device integrally with peripheral circuits formed of silicon-based devices.
For example, taking an antenna switch as an example, in the case where a high-frequency transistor for an antenna switch is formed of a compound semiconductor-based FET, it may be desirable that a CMOS decoder for RF switch control, an IPD (an integrated passive device), or the like as a peripheral circuit be formed on a different chip. In general, a high-frequency transistor for an antenna switch and a peripheral circuit are desirably built in as a module. In the case where both the high-frequency transistor for an antenna switch and the peripheral circuit are formed on chips different from each other, the manufacturing cost is increased.
Therefore, in recent years, an antenna switch device that uses an SOI (silicon on insulator) substrate allowing a CMOS decoder circuit as a silicon-based device used as a peripheral circuit to be mounted as well has been actively promoted.
Since the SOI substrate has an advantage that a parasitic capacitance is allowed to be decreased, a high-performance antenna switch device equal to the compound-based semiconductor device is achievable thereby.
However, lowered electric characteristics resulting from self-heating of the high-frequency transistor occurs.
Such self-heating occurs due to impact ionization in the vicinity of a drain end of a channel region. In particular, in the high-frequency transistor formed on the SOI substrate, a support substrate and a silicon layer are separated, for example, by silicon oxide that is a material having heat conductivity lower double digits or more than that of silicon. While heat conductivity of silicon is 144 [W/(m·k)], heat conductivity of silicon oxide is 1.1 [W/(m·k)] that is extraordinarily small.
Therefore, heat generated in the channel region is less likely to be released to a region directly under the channel. Therefore, in the SOI-type device, temperature of the device itself becomes high and electric characteristics are lowered, compared to in a bulk-type device (a device without a silicon oxide film for substrate separation between a substrate and a channel region).
With regard to an MOS transistor, a heat release structure of an SOI-type transistor such as a structure disclosed in the following Patent Literature 1 has been known. In the heat release structure, a through-hole is formed in a portion of the rear surface of the transistor of a support substrate in an SOI substrate, and a heat conductive layer made of metal is formed from the rear surface of the support substrate to the internal wall surface and the internal bottom surface of the through-hole.
With regard to a bipolar transistor, a semiconductor device having another heat release structure of an SOI-type transistor such as a structure disclosed in the following Patent Literature 2 has been known.
In the semiconductor device, on a support substrate (a first semiconductor layer) on which a substrate separation insulating layer (a first insulating layer) is formed, a second semiconductor layer (an N-type semiconductor layer, specifically, an N-type silicon layer 3 and a second oxide film 4 are formed from the substrate separation insulating layer side. Further, on the second oxide film 4, a third semiconductor layer (an N-type epitaxial layer 5) having a separation structure of an SOI-type substrate on which a device is formed is formed.
In the N-type epitaxial layer 5, an element separation insulating film is formed around a region where a transistor is formed. In the element separation insulating film, a groove that penetrates in a thickness direction to reach the N-type silicon layer 3 is formed. Undoped multicrystal silicon films 7a and 7b are buried into the groove, and thereby, a heat release-use trench 14 is formed.
With regard to improvement of high-frequency distortion characteristics, for example, a structure disclosed in the following Non-Patent Literature 1 has been known.
In the disclosed technology, with respect to an SOI substrate on which a high-frequency switch element is formed, a trench penetrating to a semiconductor substrate 101 is formed in the periphery of the foregoing element. For example, by injecting argon by an ion implantation technology, a damage layer is formed on the semiconductor substrate 101.
By allowing the damage layer to trap a carrier generated in the semiconductor substrate at the time of applying high frequency, a change in a capacity of the substrate is prevented. Further, by fixing an electric potential of the substrate at an electrode penetrating through the semiconductor substrate shown in the trench, effects of preventing the change in the capacity of the substrate is enhanced.
With regard to improvement of high-frequency distortion characteristics, a technology using a polysilicon layer as described in the following Non-Patent Literature 2 has been known.
In the technology, an SOI substrate in which a polysilicon layer is provided on a semiconductor substrate is used.
Therefore, the technology has effects of allowing undoped polysilicon to trap a carrier generated in the semiconductor substrate at the time of applying high frequency.